The COP9 complex was originally identified as a developmental switch mediating light control of plant development. Recently, it has been demonstrated that this nuclear protein complex is highly conserved among multicellular organisms. The findings that the mammalian complex possesses kinase activities and involves in regulating cellular transcription, MAP kinase pathway signaling and hormone signaling have strong implications for its critical roles in transducing environmental stimuli and regulating the cellular responses. Recently, I found that the phosphorylation of two subunits in the COP9 complex is subjected to light control, suggesting that phosphorylation, the long time speculated but never solidly proven mechanism, does play a role in light signal transduction in plants. This proposal intends to follow up this lead to demonstrate 1) the involvement of light signal in COPS complex phosphorylation and 2) the role of this phosphorylation in mediating light control of plant development. For the first goal, specific phototrceptor mutants will be utilized and their effect on COP9 complex phosphorylation will be examined. For the second goal, the phosphorylation site(s) of the two phosphorylated subunits will be identified first. Then, mutations that mimic phosphorylated and unphosphorylated forms will be made and introduced into plants to study the biological effect. The completion of the proposed works will significantly advance our current understanding of the molecular mechanism of the COP9 complex action as well as the roles of phosphorylation in light signal transduction in plants.